1. Field of the Invention
The invention relates to a ceramic electrical material with high nonlinear resistance having a base of zinc oxide and at least one other component.
The invention further is concerned with a method for producing such ceramic electrical material.
2. Description of the Prior Art
Electrical resistor materials with nonlinear current-voltage characteristics in the form of sintered ceramic masses are known in numerous compositional varieties. A main group of these materials has a zinc oxide base, to which other metal oxides are added for the formation of insulating grain boundary intermediate layers. The current-voltage characteristic of such nonlinear resistors in the range of interest is ordinarily described by the following equation: ##EQU1## I=current in mA flowing through a 1 cm.sup.2 cross section U=voltage in V across the resistor
C="nonlinear resistance" measured in V/mm in the direction of potential drop for a current of 1 mA/cm.sup.2 PA1 d=thickness in mm of the resistor in the direction of potential drop PA1 .alpha.=nonlinear (voltage -) exponent. PA1 .alpha..sub.1 for 0.1 to 1 mA/cm.sup.2 PA1 .alpha..sub.2 for 1 to 10 mA/cm.sup.2.
Customarily, .alpha. is defined for one or more current ranges of interest, e.g.:
By choice of the composition of the additives enveloping the zinc oxide base, the characteristic parameters C and .alpha. can be varied within wide limits and matched to the particular application of the resistor. In order to obtain a sufficiently large .alpha. it was thought necessary in the prior art that the mixtures contain at least one of the two oxides PbO and Bi.sub.2 O.sub.3 and still other additives for their stabilization. Such resistor materials and method of producing them are described in numerous publications (e.g. Michio Matsuoka, "Nonohmic Properties of Zinc Oxide Ceramics," Jap. Jour. Applied Physics, Vol. 10, No. 6 (June 1971); DT-OS 24 50 108; DT-AS 23 10 437; DT-OS 23 69 232).
Practically all ceramic electrical materials with a nonlinear characteristic based on zinc oxide are distinguished by a small voltage drop for given dimensions and current. For a current density of 1 mA/cm.sup.2 the field strength is 300 V/mm at most. Efforts are directed mainly at keeping this value as low as possible in order to be able to make satisfactorily small resistors for very low voltages (see e.g. DT-OS 24 50 108; DT-OS 24 45 627). In applications using resistors with such characteristics as overvoltage limiters for high voltage use, the dimensions become unacceptably large.
Most zinc oxide base nonlinear resistors have bismuth oxide as the essential additive. This is connected with the favorable effect of this component, so that there is a widespread expert prejudice to the effect that no resistor with a high nonlinear exponent .alpha. can be produced without Bi.sub.2 O.sub.3.
In practice, however, adherence to a fixed composition of the material leads to serious difficulties and the analysis of the end product can differ greatly from that of the initial mixture. This is connected with the great volatility of Bi.sub.2 O.sub.3, which at the customary sintering temperature of over 1100.degree. C. already has so high a vapor pressure that a significant portion of it evaporates during the sintering process, which leads to uncontrollable and hard-to-duplicate results in the final composition of the sintered material. The evaporation rate depends on the temperature, the time, the oven volume and the temperature gradient in the oven, and can be determined and maintained constant only with great difficulty.
Nonlinear resistor parts with a ZnO+Bi.sub.2 O.sub.3 base and containing other additives exhibit an unsatisfactory electrical stability. Their current-voltage characteristic changes during electrical loading. Such loading can consist of, for example, a d.c. current load of 1 mA/cm.sup.2 current density at 70.degree. C. ambient temperature, acting for over 500 hr. Another possible harmful type of load is, for example, a succession of two current pulses of the first standard curve shape 8/20 (interval in .mu.sec) of "IEC Publication 99-1, 1958/1970 Edition" or "VDE 0675, Guidelines for Overvoltage Protection Devices, Part 1: Valve-type Arresters for A.C. Lines of May 1972" with a maximum current density of 1000 A/cm.sup.2. Such loads alter the characteristic unfavorably in that the nonlinear resistance (C) and the nonlinear exponent (.alpha.) decrease, whereby the component involved has a reduced functional capability. It is to be noted that the characteristic becomes current-direction dependent, i.e. asymmetric; and it is no longer identical in the forward and reverse directions. This makes the component unusable for many practical applications.
From the processing point of view the desire is for the greatest possible simplification and effective control of the production process. Because of the high volatility of the additives used heretofore, the end product is dependent in its properties to a high degree on hard-to-control production parameters, whereby in particular the reproducibility of the results suffers.